1. Field of the Invention
The present invention relates to a motor-driven power steering apparatus assisting a steering force of a driver.
2. Description of the Related Art
Since a motor-driven power steering apparatus is structured such as to assist a steering force of a driver on the basis of a power of an electric motor, it is normal that a direction of a steering torque generated by a steering operation of the driver is identical to a direction of an assist torque generated by driving the electric motor, and this state is a normal state. Accordingly, when there is generated a situation that the direction of the steering torque is different from the direction of the assist torque, there can be considered an abnormality of a system of a control system constituted by a central processing unit (CPU) or the like, and there is an example (for example, refer to Japanese Patent Application Laid-Open No. 5-112251 (patent document 1)) structured such as to be provided with a second control system constituted by a sub CPU independently for coping with the situation mentioned above.
The patent document 1 discloses a control for inhibiting a drive of a motor in the case that a state in which the direction of the steering torque is different from the direction of the assist torque carries over for a predetermined time (a window time) or more.
As mentioned above, the direction of the steering torque is generally identical to the direction of the assist torque, however, there is a case of controlling in such a manner as to apply an assist torque in an inverse direction to the direction of the steering torque for holding a steering damper effect at a time of turning back the steering operation.
Since the control in the patent document 1 mentioned above makes it a condition that the control is carried over for the predetermined time or more for inhibiting the drive or the motor, it is possible to hold the steering damper effect in some degree for this predetermined time, however, in order to suitably obtain the steering damper effect in correspondence to the state, there has been separately proposed such a control as to freely vary the predetermined time on the basis of a magnitude of the steeling torque (for example, refer to Japanese Patent No. 3418098 (patent document 2)).
According to the control of the patent document 2, a long predetermined time is set for obtaining a sufficient steering damper effect at a time when the steering torque is small. If the situation occurs that the direction of the steering torque is different from the direction of the assist torque as a system abnormality, an extra drive of the motor which does not work even as a damper, carries over in this comparatively long predetermined time. Further, it appears as an unnatural behavior of the steering, particularly in the case that the torque change is large or the like.